This invention relates to shaping glass tubes for use in the production of optical fibers and, more particularly to the shaping of the end of a glass tube by means of lubricated molds.
In the manufacture of optical fibers, a process known as MCVD (modified chemical vapor deposition), which is shown and described in U.S. Pat. No. 4,217,227 of J. B. MacChesney et al is widely used. An initial step of the process involves injecting into a glass tube, commonly referred to as a starter tube, a gaseous mixture containing certain reactants while the tube is being heated by a moving torch. The particle components of the gaseous mixture, referred to as xe2x80x9csootxe2x80x9d are deposited on the interior wall of the tube in the region thereof of greatest heat. After sufficient soot is deposited, the starter tube is heated and collapsed into a glass rod from which optical fiber is subsequently drawn. During the deposition process, the remanant gases and reactants after deposition exhaust out of the end of the tube remote from the introduction end. It is not uncommon for a buildup of soot to occur at the exhaust end of the tube to the point where free flow of the mixture out of the tube is inhibited, which can affect the deposition process. As a consequence, it is the usual practice to butt fuse an exhaust tube to the exhaust end of the starter tube, the exhaust tube having a larger internal diameter to permit free flow of the exhaust gas mixture from the starter tube into and through the exhaust tube. However, if the fusion joint inside the tubes is not smooth or otherwise represents a discontinuity, there can still occur a building up of the soot at the joint which can cause clogging. It is equally as important as a smooth junction that the two tubes be precisely aligned so that they can be rotated as a unit without wobble.
It is common practice to shape the butting end of the exhaust tube by tapering its outside diameter to the diameter of the starter tube. This is usually accomplished through the use of manually manipulated graphite forming tools, such as paddles. The process requires a high level of skill on the part of the operator to the point that it deserves to be characterized as an xe2x80x9cartxe2x80x9d. Manual manipulation seldom results in any thing approximating exact duplication, hence each formed exhaust tube is, in essence, custom made. Tubes formed manually in this manner have little uniformity and dimensional commonality and, as a result, a high scrap rate of exhaust tubes is usually the case. A further common problem is that precise symmetry of the formed diameter relative to the centerline of the starter tube is quite difficult to achieve. As pointed out hereinbefore, precise alignment of the starter tube and the exhaust tube is a desideratum which is negated by any eccentricity of the exhaust tube resulting from the manual shaping process.
Some prior art methods are directed toward eliminating the use of manually manipulated paddles, by substituting therefor a mold, of graphite or other suitable material, which shapes the end of the exhaust tube, heated to ductility, while air or other fluid is applied to the interior wall surface to force the tube against the mold and to maintain the tube shape. The process requires a substantial air pressure, which tends to cool the tube to the point where the ductility thereof is insufficient for forming or shaping. In U.S. patent application Ser. No. 09/623,908; filed Jul. 31, 2000 of Jason Kay et al., the disclosure of which is incorporated herein by reference, there is shown an apparatus and method of accurately shaping the inner surface of the end portion of the exhaust tube which does not require pressurized air. The apparatus of that application includes an interior mold that has a variable configuration to allow insertion within and withdrawal from the exhaust tube.
There remain, despite the apparatus of the aforementioned Kay application, problems arising from the physical contact between the outer mold and the exhaust tube. Because the exhaust tube is rotating relative to the mold during the forming process, there is a great deal of wear of the mold which requires frequent replacement to maintain the necessary precision in the finished product. Further, the physical contact produces a distorted glass surface and, over time, undesirable irregularities therein.
The present invention is an apparatus which substantially reduces the wear of the mold and the resulting surface aberrations of the exhaust tube.
In greater detail, in a first preferred embodiment of the invention, the exterior mold for shaping the end of the rotating exhaust tube comprises first and second substantially identical members having contoured inner surfaces, and a plug for insertion into the end of the exhaust tube to maintain its internal diameter at the desired dimension and axially precise. In accordance with the present invention, each mold member has an elongated bore, preferably parallel to the axis of the assembled mold, extending from one end face thereof along substantially the entire length, but which is closed at the end of the member remote from the end face. Extending from the elongated bore, and in communication therewith, are at least two radial or transverse bores or channels which open at the interior surface of the member. Also extending from the interior surface and open thereto are one or more exhaust or vent bores offset from the radial bores and the elongated bore which connect the inner surface of the member to the exterior surface thereof. The vent bores do not intersect the elongated bore or the channels. Each of the mold members is mounted to a positioning arm for moving the member toward the central axis to form the mold and moving it away from the central axis to access the exhaust tube.
Further in accordance with the invention, the end face of each member has a suitable connection thereon for connecting the elongated bore to a fluid conduit which is, in turn, connected to a fluid (preferably water) supply for applying fluid to the elongated bore and channels. A delivery control means such as a valve member in each conduit can be used to regulate the flow rate of the water supplied to the elongated bore. In operation, the exhaust tube is mounted in, for example, a lathe chuck, and is rotated thereby. Suitable means, such as a torch, is used to heat the end of the exhaust tube to be formed to a ductile state and the mold members are moved into the mold position, embracing the end of the exhaust tube, and the mold plug is moved into place within the end of the exhaust tube. Water is applied through the conduits to the elongated bores, and through the channels to the surface of the rotating exhaust tube, thereby lubricating the mold-glass interface. The application in this manner of a water lubricant materially reduces the long term wear of the contoured inner surface of the mold members and also results in higher surface quality of the exhaust tube end.
The water or other lubricant will, when it contacts the hot glass, vaporize and the water vapor thus created will escape through the vent bores to the outside. The steady flow of water gives continuous lubrication by continuously replacing the vaporized water. The rate of such replacement is governed by the settings of the valves in the conduits, which can be accomplished manually or automatically. As the shaping step is completed, the water supply may be turned off.